The present invention relates to hip prostheses in general, and more particularly to improvements in cephalic femoral components of such prostheses.
It is already known to configurate the stem (i.e., that part which is implantable in the medullar canal of the femur) of the cephalic femoral component with relatively wide front and rear sides or surfaces which are disposed opposite each other and extend longitudinally of the stem, and with relatively narrow longitudinally extending lateral and medial sides or surfaces which also extend longitudinally of the stem and are disposed opposite each other. Such stem has a substantially rectangular outline and the width of each of the four surfaces decreases in a direction toward a rounded tip which is distal from the hip when the stem is implanted in the medullar canal of the femur. That end portion of the stem which is remote from the tip has a laterally extending neck or arm which serves to carry the ball, i.e., the male component of the hip joint which can be received in the cotyloid compartment of the hip prosthesis. The stem acts not unlike a wedge and is supposed to fit snugly into and to remain tight in the medullar canal of the femur. Such stem can be implanted with or without cementing subsequent to adequate preparation of the medullar canal. If the implantation is to take place without the use of a cement, that end portion of the stem which includes the rounded tip must constitute a wedge. Also, the width of that portion of the medial surface of the stem which is remote from the tip must increase arcuately in order to conform the outline of the shank to the shape of the adjacent portion of the femur in the calcar region. The corresponding part of the stem bears against the femur and, in the absence of highly accurate preparation of the medullar canal, can adversely influence the action of the conical supporting member for the ball fitting into the cotyloid portion of the prosthesis at that end of the stem which is remote from the rounded tip. This can result in loosening of the stem and hence in the need for remedial surgery in order to restore the operability of the artificial hip joint.
Furthermore, the very pronounced stresses which develop when a person using the just described cephalic femoral component is walking or running must be uniformly distributed to various portions of the femur, i.e., the stem and the femur should be in extensive surface-to-surface contact with one another in order to avoid deterioration of the femur as a result of excessive localized stressing of the bone tissue.